The present invention relates to methods for illuminating a Bragg cell and to a spectral analyzer employing a Bragg cell illuminated in accordance with the method disclosed herein.
The conventional manner of illuminating a Bragg cell with a cylindrically focused laser beam is to position the laser source and the Bragg cell such that the focus of the laser beam lies within the Bragg cell. This type of conventional Bragg cell illumination will be referred to herein as confocal illumination.
Conventional confocal illumination imposes limits on the dynamic operating range of the Bragg cell, and in particular imposes a lower dynamic limit on the cell.
It is an object of the present invention to provide a method for illuminating a Bragg cell which reduces the lower dynamic limit of the Bragg cell.
Another object of the present invention is to provide a spectral analyzer utilizing a Bragg cell illuminated in accordance with the method disclosed herein.
The above objects are inventively achieved in a Bragg cell which is illuminated by the steps of directing a cylindrically focused laser beam at the Bragg cell, and positioning the laser source and the Bragg cell such that the focus of the laser beam is disposed outside of the Bragg cell. A significantly reduced lower dynamic limit of the Bragg cell is thereby obtained.
A measuring set up for the dynamic range of a Bragg cell illuminated in this manner has a laser source, a beam chopper, an attenuator, beam expanding optics, a diaphragm, and a first cylinder lens all disposed in front of the Bragg cell in the direction of the beam propagation. The cylinder lens positions the focus of the laser beam slightly in front of the Bragg cell in accordance with the principles of the present invention. A detector on which the laser beam is incident is disposed after the Bragg cell, with a second cylinder lens and a Fourier lens disposed therebetween. The detector signal is lock-in-amplified. The detector is laterally positionable by a stepping motor.
The method disclosed herein of illuminating a Bragg cell with a laser beam such that the laser beam focus is outside of the Bragg cell shall be referred to herein as defocal illumination of the Bragg cell.
The method and apparatus disclosed herein proceed from consideration of light scatter which may occur during the illumination of a Bragg cell. The lower dynamic limit associated with the operation of a Bragg cell is given by the signal power NEP equivalent to the light scatter. This is a value which is proportional to light which is scattered into the signal band from the 0.sup.th order. The NEP becomes greater as the expanse of the beam in the crystal containing the Bragg cell, or the surface thereof, becomes smaller. The Bragg cell must therefore be illuminated with a beam cross-section which is as large as possible. The upper limit is determined by the size of the acoustic field in the Bragg cell, generally having dimensions of approximately 15 mm.times.0.5 mm.
Such illumination could most simply be undertaken by means of a planar wave having a cross-section fitting the size of the acoustic field of the Bragg cell, the cross-section having the aforementioned area of approximately 15 mm.times.0.5 mm. For generating such a planar wave, however, suitable optical systems such as, for example, cylinder lenses must be illuminated as well with exactly this cross-section. Strong light scatter, which is superimposed on the light scatter of the 0.sup.th order and prevents the desired reduction of the lower dynamic limit, is generated in such narrowly illuminated optical systems of cylinder lenses.
A focused laser beam however, may be produced without significant additional light scatter using a lens, that is illuminated with a large cross-section. The Bragg cell is therefore in accordance with the principles of the present invention illuminated with a focused beam with the focus of the laser beam being disposed outside of the Bragg cell, rather than in the Bragg cell as in conventional devices.